This invention relates to a system for automatically evaluating redundant process measurements and, more particularly, to an improved system for automatically detecting a signal fault among redundant signals and selecting an alternate accurate signal automatically from the redundant signals.
Single channel instrumentation and control systems can accommodate a single variable signal input. Often, two input signals, either of which can be manually selected, are provided. In the event of a signal failure, the recognition and reaction time to the failure must be sufficient to avoid serious system upsets.
In some process applications, each process parameter is monitored by two sensors to increase signal availability and to provide redundancy. Manual switching allows selection of either signal as an input to instrumentation and control system. However, automatic detection of a failed signal can be difficult to achieve. Since both sensors are monitoring the same process parameter, normally, both would indicate the same value. When one fails, however, each indicates a different value. In some cases, it is impossible to ascertain which failed without an additional standard against which the signals can be evaluated. Particularly, when the failure is a result of signal drift, i.e., drifting of the faulted signal slowly away from the correct value.